[Prolog] What is it?

[welshbyte]

Well, i know i'm going to be learning Prolog sometime next year and i know its some kind of logic programming. I wanted to get a head start so i've downloaded SWI-Prolog but now i just need to know a few things:

- What is prolog used for?
- Is it a script type of language or can you make executable files with it?
- Any code/implementation examples would be great.

I can't seem to find any "Prolog for dummies" kind of web pages around and i really just need a push to get me started past the newbie basics.

Cheers

[CSflim]

I too have just started learning prolog.

It is...um....different.

You don't type a sequence of instructions telling the program what to do, instead you give it a list of facts and rules. Prolog is apparently very useful for natural language processing, it has built in features to implement grammars for formal languages.

I have been recommended this page, I haven't really gone through it yet, but it looks pretty helpful.
Learn Prolog Now.

[welshbyte]

Thanks CSflim thats awesome

My god, there's a lot to learn here *deep breath* lets go!

[Anomaly_]

Ditto what CSFilm said. Prolog basically queries databases and attempts to match rules (known as predicates). One powerful aspect of it is that it backtracks if there are multiple matches to rules; this makes it possible to implement things concisely, albeit ineffeciently. For example, one could have a sort predicate that just generates a permutation of the list to be sorted, checks if it's sorted, and repeats the process until it is, due to backtracking.

A good example of Prolog's functionality can be illustrated in an append predicate. If we have the code

append([], L2, L2).
append([Head|Tail1], L2, [Head|Tail3]) :- append(Tail1, L2, Tail3).

in our database, we can call something like

append("abc", "def", X).

in an interactive interpreter (like SWI or GNU). This unifies X with the string "abcdef" because it is the result of appending "abc" and "def". This is simple enough, but if we instead replace the first two "parameters" of append with variables, and the result with the string "abcdef", we get all the possible lists that can comprise the string "abcdef". Here is a cut-and-paste example from the GNU Prolog interpreter:

append(X, Y, "abcdef").

X = []
Y = [97,98,99,100,101,102] ? ;

X = [97]
Y = [98,99,100,101,102] ? ;

X = [97,98]
Y = [99,100,101,102] ? ;

X = [97,98,99]
Y = [100,101,102] ? ;

X = [97,98,99,100]
Y = [101,102] ? ;

X = [97,98,99,100,101]
Y = [102] ? ;

X = [97,98,99,100,101,102]
Y = [] ? ;

The characters are translated to their ASCII values, but I think you can get the idea of what's going on. Personally, I find the language non-intuitive because it isn't algorithmic like C or Java. Nevertheless, it is very useful for analytical linguists because languages are, after all, sets of rules.

Here is one tutorial that looks helpful.

[welshbyte]

Thanks to you too Anomaly_ for the examples and opinions I think you're right about the non-intuitive thing. I've been working through the tutorial that CSflim linked to and its a pretty good one. I'll read a bit of the one you linked to too and go with the best one. Cheers

[Zarazilla]

At my University people doing AI have to program in Prolog.